The present invention relates to coaxial electrical signal cables such as are used with self-powered radiation detectors. A self-powered radiation detector consists of a central emitter, surrounded by an insulator and a conductive collector sheath about the insulator. Such detectors are used to monitor in-core radiation fluxes in power reactors for fuel management and safety applications. One of the major limitations of such self-powered detectors is the gamma induced signal current generated in the cable which connects the detector to the instrumentation panels. The cable is a coaxial electrical signal cable compatible in size with the self-powered detector, and like the detector has a central conductor surrounded by an insulator and by a conductive sheath. An electron current is generated in the coaxial signal cable as a result of being exposed to the gamma flux level present in the reactor. A relatively high background level signal induced in the cable causes degradation of the detector signal, and can also cause error in the detection of localized flux information desired for flux mapping and safety system applications.
It has been the practice to utilize a gamma compensation technique to improve the detector signal current to background current ratio in such self-powered radiation detector systems. The present state of the art is to use a second coaxial cable identical in structure and dimension to the self-powered detector and signal cable. The second cable does not have an emitter as such, but rather a conventional center conductor. The center conductors of the two cables are connected across the input terminals of a millivolt recorder. The two center conductors are connected to ground through identical load resistances, and the measured potential difference is directly proportional to the difference between the currents in the two cables. In principle, the difference should be equal to the detector current. Such gamma compensation techniques are deficient in that it is in fact difficult to match the coaxial cables such that they have identical total mass and geometry to make the compensation valid. Radiation gradients also exist within the core thimbles which can give rise to compensation signal errors. Also, a very limited area is available within the core for such self-powered detectors and the use of such compensation cables severely limits the number of actual detectors which can be utilized in an in-core assembly array.
An inherently gamma compensated cable is disclosed in U.S. Pat. No. 3,892,969, in which gamma induced currents are minimized or negated through selection of conductor and cable sheath materials and sizes. This prior art patent teaches selection of a convenient alloy such as Zircalloy as the emitter or as the central conductor of the coaxial signal cable and prescribes specific dimensions for the center cable diameter and the sheath to achieve compensation. The problem with this approach is that the prescribed dimensions may violate the mechanical constraints on the system, such as maximum size, thus making it inconvenient to use.